Vinylic Organoboranes. 4. A General, One-Pot Synthesis of 6- and 7-Alkyn-1-ols via Boracyclanes. Influence of Steric Effects in the Iodination of Lithium Alkynyl “Ate” Complexes of Dialkylborinates

Abstract

The iodination of the “ate” complexes derived from various B-alkoxyborinane derivatives and 1-alkynyllithium has been investigated. The results indicate the ate complex from B-methoxyborinane is converted into the desired 6-alkyn-1-ol in a yield of only 22%, with much larger amounts, 65%, of the undesired 1-iodo-1-alkyne. Increases in the steric bulk of the alkoxy group on boron increase the yield of the required 6-alkyn-1-ol with the best results realized with B-(triphenylmethoxy)borinane. Treatment of B- (triphenylmethoxy) borinane with 1-alkynyllithium affords the corresponding “ate” complex. Subsequent iodination induces the migration of one end of the cycloalkyl chain from boron to the adjacent carbon, resulting in the formation of the one-carbon homologated borepane moiety. This then undergoes a rapid deiodoboronation to afford the corresponding (6-alkyn-1-yl)boronate ester. Oxidation of these esters produces the desired 6-alkyn-1-ols in excellent yields (85%). An attempt to extend this reaction to di-n-alkylborinates to prepare the corresponding unsymmetrical alkynes did not achieve satisfactory results. Alternatively, the iodination of the “ate” complex from B-methylborinane and 1-alkynyllithium, followed by oxidation, provides the required 6-alkyn-1-ols in high yields. This procedure has been successfully extended to the seven-membered borepane moiety to provide the corresponding 7-alkyn-1-ols. Extension of this reaction to the di-n-alkylmethylboranes provides the corresponding unsymmetrical alkynes in good yields. Thus, these procedures constitute a simple, general and one-pot synthesis of the desired alkyn-1-ols, valuable synthons in organic synthesis. Insect pheromones, (Z)-7-tetradecenal and (Z)-7-hexadecenal, were readily prepared in excellent yields by utilizing this convenient procedure. © 1986, American Chemical Society. All rights reserved.